"What people have had available to them are primarily wood, dung and crop residues," says Catlin Powers, an environmental health researcher specializing in air pollution at Harvard University who wasn't involved in the study. "These three fuels are the most polluting fuels on earth per unit of energy extracted.

"They don't have a lot of energy, so you have to burn a lot of fuel, and that causes a lot of pollution in the process," Powers told Shots.

People in low- and middle-income countries in Southeast Asia and the Western Pacific are most affected, with 3.3 million deaths caused by indoor air pollution annually.

But it's not an easy problem to fix, despite new technologies like solar, gas and electric stoves that are more efficient and healthier than the biomass stoves many are using today.

"No matter how much you improve biomass stoves ... you can have some health benefits but you can't meet health targets," she tells Shots.

The challenges, she says, lie in distributing less-polluting stoves to people in rural areas, and getting people to want them. Many of these people sit around the stove to keep warm or use the stove to heat their beds, so more efficient stoves may not be accepted if it forces them to change those habits.

"Even if they are given the stove for free, they end up not using it," Powers says.

The bigger issue at hand is to get cleaner fuels to people, she adds, which will address not only the health hazards but also the environmental problems.

But because of population growth and increasing cost, the shift to cleaner and more efficient use of energy hasn't made much progress. In fact, the shift has slowed and even reversed, according to the International Energy Agency.